INTERNATIONAL JOURNAL ON SMART SENSING AND INTELLIGENT SYSTEMS, VOL. 4, NO. 4, DECEMBER 2011
Copper (II) oxide thin film for methanol and ethanol sensing
Mitesh Parmar and K.Rajanna
Department of Instrumentation and Applied Physics,
Indian Institute of Science,
Bangalore- 560012, India
Emails: [email protected], [email protected]
Submitted: Nov. 1, 2011
Accepted: Nov. 24, 2011
Published: Dec. 6, 2011
Abstract- A nanostructured copper (II) oxide film deposited by reactive DC-magnetron sputtering
technique, has been studied for static sensor response towards methanol and ethanol by operating
temperature and analyte concentration modulations. The optimum operating temperature (Topt) for the
sensing of methanol and ethanol is observed to be 350 ˚C and 400 ˚C, respectively. The maximum
sensitivity observed for 2500 ppm methanol and ethanol is 29% and 15.4% respectively. Another
important observation is that the sensitivity time reduces with analyte concentrations, where as
recovery time increases. The response time of 2500 ppm methanol and ethanol is 235 s and 247 s
correspondingly.
Index term: Copper (II) Oxide thin films, sputtering, gas sensing, response time and recovery time.
710
Mitesh Parmar and K.Rajanna, Copper (II) Oxide Thin Film for Methanol and Ethanol Sensing
I. INTRODUCTION
It is interesting to find the new materials and to study the material properties suitable for
the various applications, as sometimes that will be a key to overcome many challenges and
problems. During 1950s, Brattain et al. [1], Heiland [2] and Bielanski et al. [3] have initiated the
research on gas sensing by observing the effect of ambient gas on the electrical conductivity of
the materials. In 1962, further impetus to this was given by Seiyama et al. by discovering the
change in the electrical conductivity of ZnO thin film by the presence of reactive gases in the air
[4]. Since then, various types of sensing materials have been reported in the literature ranging
from SnO2, ZnO, TiO2, WO3, In2O3 to CuO, Fe2O3, NiO, and Y2O3 [5]. Recently, CuO is used to
enhance the gas sensor response of common metal oxides such as SnO2, ZnO, etc. [6−9]. The
suitability of CuO as homogeneous sensing material is one of the ongoing research problem and
in this paper we have tried to contribute in the same regard.
Although generally CuO is p-type semiconducting material [5, 10, 11] due to copper
vacancies [11], there has been reports of CuO being n-type semiconducting nature [5]. The ptype semiconducting CuO responds differently compared to normal metal oxides such as ZnO,
TiO2, SnO2 and WO3 which are n-type in nature. While observing the advantages of p-type
oxides, the temperature dependence of conduction in high-temperature range is considerably less
in the p-type oxides than that of n-types [5, 12]. Moreover, p-type oxides have tendency to
exchange lattice oxygen easily with air [5, 13]. During lifetime of the sensor, this can be useful
in maintaining stoichiometry of the oxides. These advantages in turn can be important to
maintain log-term stability of the sensor and improve the lifetime of the sensor, if used tactfully.
Considering disadvantages of p-type oxides, the most important is the mobility of their charge
carriers which can affect the sensitivity, response and recovery time of the sensor. This
disadvantage of p-type oxides can be overcome by varying the film morphology, electrode
configuration and using suitable catalyst. The use of CuO as homogeneous sensing film for CO2
sensing is first reported by Ishihara et al. [14]. As CO2 is one of the by-products during
dehydrogenation of alcohols, CuO can be used as alcohol sensing material. In the case of CuO,
the material resistance increases (instead of decreasing as in the case of n-type) during the
sensing of alcohol.
There have been reports of some interesting work based on this material, for example,
711
INTERNATIONAL JOURNAL ON SMART SENSING AND INTELLIGENT SYSTEMS, VOL. 4, NO. 4, DECEMBER 2011
work reported by Wang et al. [10]. Although the work about CuO nanorods prepared by
hydrothermal method is impressive, and shows very good response for ethanol sensing, the
sensing is observed only at 300 C. Moreover, there is no plausible reason for using 300 C as
operating temperature for the sensor. In addition to this, there are no detailed reports on methanol
sensing using CuO thin films.
In our present work, the detailed study is performed on the static sensing behavior of
reactively-sputtered nanostructured CuO film for ethanol as well as methanol by modulations of
operating temperatures and analyte concentrations. In addition to this, we are presenting the
dependency of sensing response time as well as recovery time on the analyte concentration. In
order to check the reproducibility and repeatability, all the observations are done for at least 3−5
times, and results are plotted with error-bars. The preliminary work carried out by us in this
regard has been reported elsewhere [15].
II. EXPERIMENTAL
2.1. Deposition of the films
The sensor is fabricated with nanostructured CuO as a sensing film, deposited over 1μm
thick SiO2 coated Si substrate, using reactive DC-magnetron sputtering technique. Silver contact
films on either end of the sensing film are deposited in order to get two-end electrode
configuration. As the Ag electrodes are fabricated over the sensing films are 6 mm apart, the
catalytic activities due to diffusion of Ag into CuO sensing material, can be neglected. In order to
avoid the oxidization of Ag during the sensing, 10 nm gold film is used as protecting layer. Fig.
1 shows the schematic diagram of the sensor.
The sensor consists of dc-magnetron sputtered nanostructured CuO sensing film over
comb-type silver electrode deposited over 1µm thick SiO2 coated Si substrate. The contact films
on either end of the sensing film are deposited in order to get two-end electrode configuration.
As the Ag electrodes are fabricated over the sensing films are 6 mm apart, the catalytic activities
due to diffusion of Ag into CuO sensing material, can be neglected. Fig. 1 shows the schematic
diagram of the sensor.
712
Mitesh Parmar and K.Rajanna, Copper (II) Oxide Thin Film for Methanol and Ethanol Sensing
Figure 1. Schematic diagram of the sensor.
The optimized sputtering process parameters for the deposition of Au, Ag and CuO films,
using gold, silver and Oxygen Free High Conductive (OFHC) copper target materials, are given
in Table 1. Pre-sputtering of these targets is done appropriately for some time, in order to remove
the surface impurities. The thicknesses of the films are measured by the surface profilometer –
Taylor Hobson - Form Talysurf Plus.
Table 1: Optimized sputtering process parameters for the deposition of CuO, Ag and Au films.
Sputtering
Parameter
Materials deposited
CuO
Ag
Au
Working distance (mm)
55
55
52
Substrate Temperature (˚C)
375
Room Temp.
Room Temp.
1 x 10-6
3.7 x 10-6
8 x 10-6
90:10
NA
NA
Working pressure (mbar)
3 x 10-2
3.5 x 10-2
8 x 10-3
Current density (mA/cm2)
0.679
0.51
0.334
Deposition rate (nm/minute)
5.67
18
50
85
180
≈10
Ultimate Vacuum (mbar)
Argon-Oxygen Ratio
Film thickness (nm)
713
INTERNATIONAL JOURNAL ON SMART SENSING AND INTELLIGENT SYSTEMS, VOL. 4, NO. 4, DECEMBER 2011
2.2. Characterization of the film
The crystal structure of the sensing sample is determined by X-ray diffraction (XRD)
using Bruker D8 Advance X-ray diffractometer with Cu Kα radiation of wavelength 1.54 Ǻ. The
XRD spectrum is recorded in the range, 2θ = 20°–50°. For the XPS analysis, SPECS GmbH
spectrometer (Phoibos 100MCD Energy Analyzer) using MgKα radiation (1253.6 eV) and
CASA XPS analysis software is used. The morphologies of the sensing film is investigated using
FEI Quanta 200 Environmental scanning electron microscope (ESEM).
2.3. Design of experimental set-up to study sensing behaviour and the sensing procedure
The gas sensing measurements is done with the help of in-house designed testing set-up
as shown in Fig. 2. The sensor sample is placed over a heater (accuracy ±1 C) located inside the
chamber, and pressure-contacts are used for monitoring electrical behaviour.
Figure 2. Schematic diagram of the in-house designed experimental set-up.
The sensor is heated at 200 C and subsequently degased using rotary pump. After this,
the sample is maintained at the operating temperature that needs to be tested and the zero-air (air
with moisture ≤ 5 ppm) is admitted to the chamber in order to create atmospheric pressure. The
volume for the corresponding ppm concentration of alcohols is calculated using the following
equation [16];
C (ppm) = [10 C1V1dRT]/[P0VcM]
714
(1)
Mitesh Parmar and K.Rajanna, Copper (II) Oxide Thin Film for Methanol and Ethanol Sensing
where C1 is concentration in the liquid analyte (wt. %), V1 is the injected volume of liquid (ml),
d is the density of the analyte (gm/ml), R is the universal gas constant (l.atm / K.mol), T is the
temperature (K), P0 is the pressure in the chamber (atm), Vc is the chamber volume (l) and M is
the molecular weight (gm / mol), respectively.
As some of the values in the equation-1 are constant, the concentration of analyte at room
temperature (298 K) can be given as C (ppm) = [C1V1d×244.66]/[P0VcM]
(2)
V1 (mL) = [P0VcMC]/[C1V1d×244.66]
(3)
For our case, the volume of the chamber, Vc = 1.35 liter and the pressure maintained in the
chamber, P0 = 1 atm.
The entire static-sensing studies carried out for both methanol and ethanol separately,
consist of two parts. In the first part, the sensing behavior of the films are studied for the
sensitivity of fixed concentrations of alcohols by temperature modulation to find the optimum
sensing temperature (Topt), whereas in the second part, the alcohol sensitivity for different
concentrations is studied at Topt. In the present work, the alcohol analytes are Methanol
(AR.99.9%, Sisco Research Laboratories Pvt. Ltd., India) and ethanol (AR.99.9%, Changshu
Yangyuan Chemicals, China). The details of these experimental results are discussed in the
following section.
III. RESULTS AND DISCUSSION
3.1. Characterization of sensing film
The XRD analysis is primarily used to confirm the sensing film material and to determine
the structural orientation of sputtered-CuO film. Fig. 3 show XRD spectrum of annealed (550 C,
2 h) as-deposited CuO sensing film. Although the background diffraction peaks of Si (100)
substrate exist, three major diffraction peaks of CuxOy are observed. These observed peaks with
2θ values of 28.1°, 35.6° and 47.7° corresponds to Cu16O14.15 (112), CuO (002/-111) and
Cu16O14.15 (301) respectively. The CuO peak gives the highest intensity among them. Using
Scherrer formula [17−18] grain size is calculated to be in the range of 52 nm.
715
INTERNATIONAL JOURNAL ON SMART SENSING AND INTELLIGENT SYSTEMS, VOL. 4, NO. 4, DECEMBER 2011
Figure 3. X-ray diffraction spectrum of annealed (550 °C, 2 h) as-deposited CuO sensing film.
The sensing film is analyzed using XPS in order to verify the stoichiometry of the film.
Fig. 4(a)-(b) shows the spectra for copper and oxygen in CuO film. The peak for Cu (2P3/2) at
binding energy of 933.88 eV showed the resemblance with the observation reported by Fleisch et
al. [19] and Mclntyre et al. [20]. Also, O (1S) peak at binding energy of 529.99 eV shows the
resemblance with the results of Hirokawa et al. [21].
Figure 4. Core level XPS spectra of as-deposited CuO film (a) Cu 2p3/2 and (b) O1s.
716
Mitesh Parmar and K.Rajanna, Copper (II) Oxide Thin Film for Methanol and Ethanol Sensing
The SEM image of the as-deposited CuO film is shown in Fig. 5. When the grain size
during FESEM imaging is measured, the film consists of nanostructures with size distribution of
40-65 nm. This size distribution can be attributed to high substrate temperature during the
deposition of film. As small size of nanostructures increases surface area to volume ratio, the
reactivity of the material increases, which in turn, will enhance the sensitivity of the sensing film.
In addition to this, the grain boundaries between these nanostructures provide the resistance
barrier for the charge carriers, hence increases the resistance of the film.
Figure 5. FESEM image of the as-deposited CuO film.
3.2. Response of sensing film for Methanol and Ethanol
As discussed above, the sensing film is kept at 200 ˚C in a closed chamber and degased
using rotary pump. In order to create atmospheric pressure inside the chamber the zero-air is
admitted in the chamber. Consequently, it has been observed that since CuO is a p-type semiconducting material its resistance decreases and tends to be constant in approximately 3 minutes.
The film resistance at that point of time is taken as initial/base resistance (Rair – resistance in the
presence of air). Subsequently, when certain concentration of an methanol/ethanol is introduced
in to the chamber, it is chemisorbed on the surface of the CuO film. Using pre-adsorbed oxygen
atom, ethanol undergoes dehydrogenation in order to breakdown CO2 and H2. During this
dehydrogenation, it releases electrons into the film thereby increasing the number of minority
charge carriers and reducing majority charge carriers in the p-type semiconducting film. This in
turn, increases the resistance of the sensing film. The change in the film resistance saturates at
717
INTERNATIONAL JOURNAL ON SMART SENSING AND INTELLIGENT SYSTEMS, VOL. 4, NO. 4, DECEMBER 2011
some point of time (Rg – resistance after the admittance of gas/analyte) depends upon the factors
such as operating temperature, sensing material, active sensing area, concentration of the analyte,
contact electrode patterns, etc. The sensitivity was calculated using the expression given in the
literature [22]:
Sensitivity = [(Rgas – Rair)/Rair] × 100% (4)
One of the important things to consider here is the effect of operating temperature on the
nanostructured sensing film. Once the sensor is tested at maximum operating temperature (450
˚C) the film morphology tends to vary due to agglomeration. This leads to variation in the
sensing response when measured again at lower temperature. In order to nullify the effect of
varying operating temperature during the sensing, the film is annealed at 550 C (i.e. maximum
operating temperature (450 C) + 100 C) for 2 h.
3.2.1. Variation in the base resistance of the sensor as a function of operating temperature
The sensing response is measured using the electrometer, at fixed bias voltage of 10 V.
As shown in Fig. 6, the film base resistance measured during the ethanol (500 ppm) sensing at
varying operating temperature is observed to be 227 KΩ at 100 ˚C and subsequently decreased to
300 Ω at 500 ˚C. The base resistance measured during the methanol (500 ppm) sensing is almost
equivalent to the base resistance values during ethanol sensing.
Figure 6. Variation in the base resistance of the sensing film with operating temperature
modulation.
718
Mitesh Parmar and K.Rajanna, Copper (II) Oxide Thin Film for Methanol and Ethanol Sensing
3.2.2. Optimization of the operating temperature for methanol and ethanol sensing
The sensitivity variation for methanol and ethanol with temperature modulation has been
shown in fig. 7 (a) and (b). It may be noted that these temperatures 350 ˚C and 400 ˚C are the
optimized operating temperatures (Topt) for methanol and ethanol respectively, where the
sensitivity is the maximum. In order to confirm the repeatability of the results, the sensitivity is
observed at different concentrations of 300-500-700 ppm for both the analytes for at least 3-5
times. The maximum sensitivity for 700 ppm of methanol and ethanol (separately) is observed to
be 17% and 8.3% respectively. In the case of two-end electrode configuration the possibility of
detection of induced charge carrier is quite low and the chances of recombination are high due to
large inter-electrode distance (6 mm in present case) and mobility of charge carriers in CuO. The
results obtained in the case of methanol and ethanol sensing, further validates the results reported
by Cordi et al. [23]. According to their observation, the dehydrogenation of CH3OH/C2H5OH
results in the formation of H/H2O and CO/CO2. In our case, the formation of CO2 during the
dehydrogenation indicates the complete breakdown of the ethanol. CO formed during this
process may get converted into CO2 using one oxygen atom. This oxygen atom might be preadsorbed over the sensing surface or can be from the CuO lattice. This results in partial reduction
of CuO film into Cu+ film varying the stoichiometry of the film. The major advantage of
methanol/ethanol sensing from 350–400 C is the oxidation of reduced sensing film which
facilitates the easy recovery of the CuO sensing material [10, 24, 25]. This will help to maintain
the stoichiometry. This will ultimately improve the reliability and shelf-life of the sensor.
Moreover, sensing at very high temperature has advantage of nullifying the moisture effect.
The observed lower Topt in the case of methanol compared to ethanol is believed to be
due to its higher vapor pressure and thus lower energy is required to undergo complete
dehydrogenation. Fig. 7 – (c) and (d) shows the resistance change during methanol and ethanol
sensing at their Topt, respectively. During complete sensor testing procedure, the base resistance
varies ≤1%. This indicates that the sensing process is reversible with steady base resistance
value.
719
INTERNATIONAL JOURNAL ON SMART SENSING AND INTELLIGENT SYSTEMS, VOL. 4, NO. 4, DECEMBER 2011
Figure 7. Sensitivity verses temperature graph of the CuO film for (a) methanol and (b) ethanol,
respectively. (c−d) Resistance change during methanol and ethanol at respective Topt.
3.2.3. Sensitivity with varying analyte concentrations
In order to study the sensor behavior by varying concentrations of methanol and ethanol,
the sensitivity is further observed by concentration modulations (100 ppm to 2500 ppm) by
maintaining their respective Topt (Fig. 8). The figure also contains in-set graph showing the
magnified version of the variation of the sensitivity versus analyte concentrations (with step-size
of 100 ppm) for the concentration range 100 – 900 ppm. Although the sensitivity is linear for
lower concentrations (for 100 to 900 ppm), it indicates a tendency of sensitivity saturation at
higher concentrations (2500 ppm). The maximum sensitivity observed for 2500 ppm of methanol
and ethanol is 29% and 15.4% respectively, and the maximum error-bar size is 0.25% and 0.16%
of full scale for methanol and ethanol. The possible reason for this behavior might be the
fixed/limited availability of active sensing area. When the analyte concentrations are low (≤900
720
Mitesh Parmar and K.Rajanna, Copper (II) Oxide Thin Film for Methanol and Ethanol Sensing
ppm), the proper sensing is possible as analyte molecules will form a monolayer over sensing
film in order to be chemisorbed. This results into their complete chemisorption. On the contrary
at higher concentrations (≈2500 ppm), the analyte molecules left-over after the monolayer
formation will not reach the sensing film in order to be chemisorbed. Hence, the chemisorption
rate is limited due to limited active sensing area which in turn affects the sensitivity. The similar
trends have been observed during both the analytes.
Figure 8. Sensitivity verses Methanol concentration and ethanol concentration at the operating
temperature of 350 ˚C and 400 ˚C respectively.
3.2.4. Response time and recovery time as a function of analyte concentrations
In addition to the above, the other important observation is regarding the response time
and recovery time of the sensing film. The response time is the duration by which the sensor
response reaches to almost 90% of the saturation value, whereas the recovery time is the duration
by which the sensor response reaches from 90% to almost 10% of the saturation value. The
sensing response time and recovery time depends upon the operating temperature of the sensor,
the analyte type and their concentrations. Fig. 9(a−b) shows the effect of analyte type and
varying concentrations of analyte on the response time and recovery time at the optimum
operating temperatures of the analytes (350 ˚C for methanol and 400 ˚C for ethanol).
From fig.9(a), it is evident that the sensing response time depends on the type of analytes
and the analyte concentration. The response time of 2500 ppm methanol and ethanol is 235 s and
247 s correspondingly, and the maximum size of error-bar is of 18 s. The higher response time
721
INTERNATIONAL JOURNAL ON SMART SENSING AND INTELLIGENT SYSTEMS, VOL. 4, NO. 4, DECEMBER 2011
for ethanol can be due to higher molecular weight of ethanol as compared to methanol, and
hence, takes longer time to undergo dehydrogenation. In the case of varying concentration, the
sensing response time is proportional to the analyte concentration, and tends to saturates at
higher concentrations (≈2500 ppm). This can be attributed to the limited/fixed active sensing
area compared to the analyte concentration, hence lower rate of chemisorption or/and higher
probability of inter-molecular collision of the analyte.
Figure 9. Effect of analyte concentration modulation on sensing response time and recovery time
(a) methanol and (b) ethanol.
Fig.9(b) shows the dependency of recovery time on analyte concentration. The recovery
time for both the analytes seems to be linear during lower concentrations (≤900 ppm) and tends
to saturates at higher concentrations (≥1000 ppm). The recovery time for 100 ppm methanol and
ethanol is 235 s and 244 s respectively, and the maximum error-bar size is found to be 12 s for
2500 ppm of both the analytes. When the analyte concentrations are low (≤900 ppm), analyte
molecules will form incomplete/complete monolayer over sensing film in order to be
chemisorbed. Due to this chemisorption, the recovery time for the sensor increases linearly with
lower concentration. On the contrary at higher concentrations (≈2500 ppm), the analyte
molecules left-over after the complete monolayer formation will not reach the sensing film in
order to be chemisorbed. Hence, the recovery time tends to saturates at higher concentration. The
similar trends have been observed during both the analytes.
722
Mitesh Parmar and K.Rajanna, Copper (II) Oxide Thin Film for Methanol and Ethanol Sensing
IV. CONCLUSIONS
On the basis of the experimental techniques and the discussion of results in this paper, the
possible applicability of nanostructured copper (II) oxide films for the methanol and ethanol
sensing have been demonstrated. The optimum operating temperature for sensing of methanol
and ethanol in the case of the CuO film is found to be 350 ˚C and 400 ˚C respectively, providing
better sensitivity and selectivity. The sensitivity is found to be linear for lower concentrations
and tends to saturate at higher concentrations. In addition to this, it has been concluded that the
sensing response time also depend on the analyte concentration. This performance study on the
CuO film will be helpful for the optimization of various important parameters, such as operating
temperature and response time, leads to full-fledged methanol/ethanol sensor using
nanostructured CuO as sensing film and might lead to whole new range of sensors using p-type
semiconducting sensing material.
ACKNOWLEDGMENT
Authors wish to thank the Institute Nanoscience Initiative and the Central XRD facility at
the institute campus for their kind help. Thanks are also to the Plasma processing lab at our
department for XPS facility.
REFERENCES
[1]
W. Brattain and J. Bardeen, “Surface properties of Germanium”, Bell Syst. Tech. J., Vol.
32, 1953, pp. 1–41.
[2]
G. Heiland, “Zum Einfluss von adsorbiertem Sauerstoff auf die elektrische Leitfähigkeit
von Zn-O-Kristallen”, Z. Physik, Vol. 138, 1954, pp. 459–464.
[3]
A. Bielanski, J. Derren and J.Haber, “Electric conductivity and catalytic activity of
semiconducting oxide catalysts”, Nature, Vol. 179, 1957, pp. 668–669.
[4]
T. Seiyama, A. Kato, K. Fujiishi and M. Nagatani, “A new detector for gaseous
components using semiconducting thin films”, Anal. Chem., Vol. 34, 1962, pp. 1502–
1503.
[5]
G. Korotcenkov, “Metal oxides for solid-state gas sensors: What determines our
723
INTERNATIONAL JOURNAL ON SMART SENSING AND INTELLIGENT SYSTEMS, VOL. 4, NO. 4, DECEMBER 2011
choice?”, Mater. Sci. Eng. B, Vol. 139, 2007, pp. 1–23.
[6]
A. Chowdhuri, V. Gupta and K. Sreenivas, “Fast response H2S gas sensing characteristics
with ultra-thin CuO islands on sputtered SnO2”, Sens. Actuators B, Vol. 93, 2003, pp.
572–579.
[7]
X. Zhou, Q. Cao, H. Huang, P. Yang and Y. Hu, “Study on sensing mechanism of CuO–
SnO2 gas sensors”, Mater. Sci. Eng. B, Vol. 99, 2003, pp. 44–47.
[8]
S. T. Jun and G. M. Choi, “CO gas-sensing properties of ZnO/CuO contact ceramics”,
Sens. Actuators B, 17, Vol. 1994, pp. 175–178.
[9]
K. K. Baek and H. L. Tuller, “Atmosphere sensitive CuO/ZnO junctions”, Solid State
Ionics, Vol. 75, 1995, pp. 179–186.
[10]
C. Wang, X.Q. Fu, X. Y. Xue, Y. G. Wang and T. H. Wang, “Surface accumulation
conduction controlled sensing characteristic of p-type CuO nanorods induced by oxygen
adsorption”, Nanotechnology, Vol. 18, 2007, pp. 145506–14510.
[11]
J. Bardeen, W. H. Brattain and W. Shockley, “Investigation of Oxidation of Copper by
Use of Radioactive Cu Tracer”, J. Chem. Phys., Vol. 14, 1946, pp. 714.
[12]
P. T. Moseley, “Solid state gas sensor”, Meas. Sci. Technol., Vol. 8, 1997, pp. 223–237.
[13]
M. J. Madou and S. R. Morrison, “Chemical Sensing with Solid State Devices”,
Academic Press, Inc./Harcourt Brace Jovanovich Publ., Boston, NY, 1987.
[14]
T. Ishihara, M. Higuchi, T. Takagi, M. Ito, H. Nishiguchi and Y. Takita, “Preparation of
CuO thin films on porous BaTiO3 and application as a CO2 sensor”, J. Mater. Chem.,
Vol. 8(9) 1998, pp. 2037–2042.
[15]
M. Parmar, N. Gokhale and K. Rajanna, “Nanostructured Copper(II) oxide thin film for
alcohol sensing”, Proceedings of IEEE International Instrumentation Measurement
Technology Conference (I2MTC)– 2009 pp. 337–340.
[16]
T. Nakamoto, M. Yosihioka, Y. Tanaka, K. Kobayashi, T. Moriizumi, S. Ueyama and W.
S. Yerazunis, “Colorimetric method for odor discrimination using dye-coated plate and
multiLED sensor”, Sens. Actuators B, Vol. 116, 2006, pp. 202–206.
[17]
P. Scherrer, “Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen”, 1918,
pp. 98−100.
[18]
A. Patterson, “The Scherrer formula for X-Ray particle size determination”, Phys. Rev.,
Vol. 56, 1939, pp. 978−982.
724
Mitesh Parmar and K.Rajanna, Copper (II) Oxide Thin Film for Methanol and Ethanol Sensing
[19]
T. H. Fleisch and G. J. Mains, “Reduction of copper oxides by ultraviolet radiation and
atomic hydrogen studied by XPS”, Appl. Surface Sci., Vol. 10, 1982, pp. 51–62.
[20]
N. S. Mclntyre and M. g. Cook, “X-ray photoelectron studies on some oxides and
hydroxides of cobalt, nickel and copper”, Anal. Chem., Vol. 47, No. 13, 1975, pp. 2208–
2213.
[21]
K. Hirokawa, F. H. Honda and S. M. Oku, “On the surface chemical reactions of metal
and oxide XPS samples at 300–400° at a high vacuum produced by oil diffusion pumps”,
J. Electron Spectrosc., Vol. 6, 1975, pp. 333–345.
[22]
N.S. Ramgir, S. Kailasa Ganapathi, M. Kaur, N. Datta, K. P. Muthe, D. K. Aswal, S. K.
Gupta and J. V. Yakhmi, Sub-ppm H2S sensing at room temperature using CuO thin
films, Sens. Actuators B, Vol. 151, 2010, pp. 90–96.
[23]
E. M. Cordi, P. J. O’Neill and J. L. Falconer, “Transient oxidation of volatile organic
compounds on a CuO/Al2O3 catalyst”, Appl. Catal. B, Vol. 14, 1997, pp. 23–36.
[24]
V. Figueiredo, E. Elangovan, G. Concalves, P. Barquinha, L. Pereira, N. Franco, E.
Alves, R. Martins, and E. Fortunato, “Effect of post-annealing on the properties of copper
oxide thin films obtained from the oxidation of evaporated metallic copper”,
Applied Surface Science, Vol. 254, 2008, pp. 3949‐3954.
[25]
L. Zhou, S.Günther, D. Moszynski, and R. Imbihl, “Reactivity of oxidized copper
surfaces in methanol oxidation”, J. Catalysis, Vol. 235, No. 2, 2005, pp. 359–367.
725